Guanine Oxidation Research Articles

The R- and S-diastereoisomeric effects on the guanidinohydantoin-induced mutations in DNA.

Direct and indirect oxidation of guanine in DNA produces guanidinohydantoin (Gh), which is capable of inhibiting replication and inducing mutations during cellular activities. Although some biochemical studies have proposed that Gh may induce exclusively G to C mutations in DNA, other studies have predicted the occurrence of both G to C and G to T mutations. However, the exact reasons for these mutations and the dubious character of Gh in this context are not yet understood. Further, due to insufficient structural data, the electronic structure of Gh that can participate in the formation of different base pair complexes in DNA is also not known. Here, density functional theory (DFT) is used to find the most stable tautomers of Gh at the base level out of a total 112 possible tautomers and their involvement in mutagenesis is investigated by computing structures, energies and electronic properties of different base pair complexes formed between the syn- and anti-conformations of the most stable tautomer of Gh (aGh) and all the bases of DNA. It is found that aGh can coexist in R- and S-diastereoisomeric configurations. Due to the flexible guanidinium group, it can rotate about the N3-C4 bond in each of the above diastereoisomers to form two different stable conformations (aGh1 and aGh2). It is further shown that among the different base pair complexes involving aGh1, syn-aGh1:G is the most stable. This indicates that G would be easily incorporated against syn-aGh1 giving rise to G to C mutations in DNA. However, in the case of aGh2, G is the preferred base pair partner of syn-aGh2 and T is the preferred base pair partner of anti-aGh2. This implies that in addition to G to C mutations, the occurrence of aGh2 in DNA may also induce G to A mutations. Further, due to similarities between base pairing patterns and binding energies of syn-aGh1:A and syn-aGh2:A complexes with those of the T:A complex, DNA polymerases may mistakenly insert A opposite aGh1 or aGh2 by misrecognizing the latter as T. This may ultimately induce G to T mutations in DNA. However, as the constraints imposed by the DNA backbones and stacking interactions were not considered here, the possibilities of aGh2:T and aGh2:A base pairs need to be investigated experimentally. It is further found that the mutagenic character of aGh in the R- and S-diastereoisomeric forms is similar.

Label-Free Electrochemical DNA Biosensor for the Detection of Mycobacterium Tuberculosis Using Gold Electrode Modified by Self-Assembled Monolayer of Thiol

Tuberculosis (TB), an infectious disease affects millions of humans worldwide and is caused by the Mycobacterium tuberculosis. A label free electrochemical DNA biosensor for the detection of M. tuberculosis using gold electrode modified by self-assembled monolayer of thiol is presented in this paper. Single-stranded DNA probe was immobilized on the surface of self-assembled monolayer gold electrode with the assistance of cysteamine and glutaraldehyde, which was further used to hybridize with the target sequence and non-complementary target sequence. Differential Pulse Voltammetry (DPV) was used to characterize the self-assembled monolayer on the gold electrode and also to study the immobilization of ssDNA probe and hybridization with the complementary sequence (target ssDNA). The hybridization reaction on the gold electrode surface was detected by monitoring a guanine oxidation signal at potential +0.21V. Electrochemical DNA biosensor using gold electrode modified of thiol (Au-SAM) can be used to determine hybridization between ssDNA probe and ssDNA target sequence of M. tuberculosis with sensitvity value is 0.5152; detection limit is 3.47μg.mL-1 and quantification limit is 11.56μg.mL-1

Electrochemically Reduced Carboxyl Graphene Modified Electrode for Simultaneous Determination of Guanine and Adenine

An electrochemically reduced carboxyl graphene modified glassy carbon electrode (ERCGr/GCE) was prepared from a carboxyl graphene modified glassy carbon electrode (CGr/GCE) and employed for the simultaneous determination of guanine and adenine. The ERCGr/GCE showed an enhanced voltammetric response toward the oxidation of guanine and adenine compared with the CGr/GCE because the conductivity and electrochemical active surface area increased during the reduction process. The voltammetric peak current was linearly dependent on guanine and adenine concentration over the ranges of 0.5–10 and 2.5–50 µmol L−1, respectively. The detection limits were 0.15 µmol L−1 for guanine and 0.10 µmol L−1 for adenine in 50 µmol L−1 phosphate buffer at pH 6.86. Determination of guanine and adenine in thermally denatured herring sperm DNA showed that the ratio of guanine/adenine was 0.758 demonstrating practical application of the ERCGr/GCE.

Oxidatively generated damage to cellular DNA by UVB and UVA radiation.

This review article focuses on a critical survey of the main available information on the UVB and UVA oxidative reactions to cellular DNA as the result of direct interactions of UV photons, photosensitized pathways and biochemical responses including inflammation and bystander effects. UVA radiation appears to be much more efficient than UVB in inducing oxidatively generated damage to the bases and 2-deoxyribose moieties of DNA in isolated cells and skin. The UVA-induced generation of 8-oxo-7,8-dihydroguanine is mostly rationalized in terms of selective guanine oxidation by singlet oxygen generated through type II photosensitization mechanism. In addition, hydroxyl radical whose formation may be accounted for by metal-catalyzed Haber-Weiss reactions subsequent to the initial generation of superoxide anion radical contributes in a minor way to the DNA degradation. This leads to the formation of both oxidized purine and pyrimidine bases together with DNA single-strand breaks at the exclusion, however, of direct double-strand breaks. No evidence has been provided so far for the implication of delayed oxidative degradation pathways of cellular DNA. In that respect putative characteristic UVA-induced DNA damage could include single and more complex lesions arising from one-electron oxidation of the guanine base together with aldehyde adducts to amino-substituted nucleobases.

Electrochemical determination of the anticancer drug taxol at a ds-DNA modified pencil-graphite electrode and its application as a label-free electrochemical biosensor

In this study a novel biosensor for determination of taxol is described. The interaction of taxol with salmon-sperm double-stranded DNA (ds-DNA) based on the decreasing of the oxidation signals of guanine and adenine bases was studied electrochemically with a pencil-graphite electrode (PGE) using a differential pulse voltammetry (DPV) method. The decreases in the intensity of the guanine and adenine oxidation signals after interaction with taxol were used as indicator signals for the sensitive determination of taxol. DPV exhibits a linear dynamic range of 2.0×10−7–1.0×10−5M for taxol with a detection limit of 8.0×10−8M. Finally, this modified electrode was used for determination of taxol in some real samples.

Ultra-sensitive detection of leukemia by graphene.

Graphene oxide nanoplatelets (GONPs) with extremely sharp edges (lateral dimensions ∼ 20-200 nm and thicknesses <2 nm) were applied in extraction of the overexpressed guanine synthesized in the cytoplasm of leukemia cells. The blood serums containing the extracted guanine were used in differential pulse voltammetry (DPV) with reduced graphene oxide nanowall (rGONW) electrodes to develop fast and ultra-sensitive electrochemical detection of leukemia cells at leukemia fractions (LFs) of ∼ 10(-11) (as the lower detection limit). The stability of the DPV signals obtained by oxidation of the extracted guanine on the rGONWs was studied after 20 cycles. Without the guanine extraction, the DPV peaks relating to guanine oxidation of normal and abnormal cells overlapped at LFs <10(-9), and consequently, the performance of rGONWs alone was limited at this level. As a benchmark, the DPV using glassy carbon electrodes was able to detect only LFs ∼ 10(-2). The ultra-sensitivity obtained by this combination method (guanine extraction by GONPs and then guanine oxidation by rGONWs) is five orders of magnitude better than the sensitivity of the best current technologies (e.g., specific mutations by polymerase chain reaction) which not only are expensive, but also require a few days for diagnosis.

Metal-organic frameworks and β-cyclodextrin-based composite electrode for simultaneous quantification of guanine and adenine in a lab-on-valve manifold.

In this work, a novel chemically modified electrode is constructed based on metal-organic frameworks and β-cyclodextrin (Cu3(BTC)2/β-CD, BTC = benzene-1,3,5-tricarboxylate) composite material. The electrode was used for simultaneous determination of guanine and adenine in a sequential injection lab-on-valve format and exhibited sensitive responses to guanine and adenine oxidation due to the π-π stacking interaction of Cu3(BTC)2 and the inclusion behavior of β-CD. The analytical performance was assessed with respect to the supporting electrolyte and its pH, accumulation time and accumulation potential, and the fluid flow rates. Under optimal conditions, linear calibration ranges for both guanine and adenine were from 1.0 × 10(-7) to 1.0 × 10(-5) mol L(-1), and detection limits (S/N = 3) were found to be 5.2 × 10(-8) and 2.8 × 10(-8) mol L(-1), respectively. The proposed sensor showed advantages of high sensitivity, simple sample preparation protocol, enhanced throughput and good reproducibility. Finally, the practical application of the proposed sensor has been performed for the determination of guanine and adenine in real samples with satisfactory results.

Gelatin methacrylate (GelMA) mediated electrochemical DNA biosensor for DNA hybridization

In this study, an electrochemical biosensor system for the detection of DNA hybridization by using gelatin methacrylate (GelMA) modified electrodes was developed. Electrochemical behavior of GelMA modified Pencil Graphite Electrode (PGE) that serve as a functional platform was investigated by using Cyclic Voltammetry (CV) and Electrochemical Impedance Spectroscopy (EIS) and compared with those of the bare PGE. Hybridization was achieved in solution phase and guanine oxidation signal changes were evaluated. The decrease in the guanine oxidation peak currents at around +1.0V was used as an indicator for the DNA hybridization. Also, more interestingly GelMA intrinsic oxidation peaks at around +0.7V changed substantially by immobilization of different oligonucleotides such as probe, hybrid and control sequences to the electrode surface. It is the first study of using GelMA as a part of an electrochemical biosensor system. The results are very promising in terms of using GelMA as a new DNA hybridization indicator. Additionally, GelMA modified electrodes could be useful for detecting ultra low quantity of oligonucleotides by providing mechanical support to the bio-recognition layer. The detection limit of this method is at present 10-12mol. Signal suppressions were increased from 50% to 93% for hybrid with using GelMA when it was compared to bare electrode which facilitates the hybridization detection.

Autonomous exonuclease III-assisted isothermal cycling signal amplification: a facile and highly sensitive fluorescence DNA glycosylase activity assay.

One common form of DNA damage is the oxidation of guanine to 8-oxo-7,8-dihydroguanine (8-oxoG), which can be carcinogenic. Human 8-oxoguanine DNA glycosylase (hOGG1) is a key base excision repair (BER) enzyme that repairs 8-oxoG, and the expression level of hOGG1 is closely related to many types of human cancers. Herein, a novel and highly sensitive fluorescence biosensing platform for hOGG1 activity detection has been constructed based on autonomous exonuclease III (Exo III)-assisted signal amplification. Two hairpin probes (HP1 and HP2) are ingeniously designed. In the presence of hOGG1, HP1 is cleaved at the 8-oxoG site, and the stem is subsequently digested by Exo III, releasing the trigger DNA fragment (tDNA1). Successively, tDNA1 partially hybridizes with HP2 to initiate the Exo III-assisted cycling cleavage to release another trigger DNA fragment (tDNA2), which in turn triggers the cycling cleavage of DNA fluorescence probe (FP). Therefore, large amount of fluorophore fragments are released, leading to a significantly amplified fluorescence signal toward hOGG1 activity detection. A directly measured detection limit down to 0.001 U/mL is obtained, which is much lower than that of the approaches reported in literature. In addition to high sensitivity and good selectivity, the as-proposed strategy also exhibits the advantages of isothermal experimental condition, simplicity, and convenience. Furthermore, the Exo III-assisted autonomous cycling cleavage approach we proposed here is a universal sensing strategy and has great potential in assays of many other biological analytes.

Spongy graphene electrode in electrochemical detection of leukemia at single-cell levels

Mg2+-charged spongy graphene electrodes (SGEs) were fabricated by using electrophoretic deposition of chemically exfoliated graphene oxide sheets on graphite rods. The SGEs were able to present two distinguishable signals (originated from electrochemical oxidation of guanine) in differential pulse voltammetry (DPV) of leukemia and normal blood cells, in contrast to glassy carbon electrodes giving only one overlapped peak. Hence, the SGEs were applied in fast (60min) and ultra sensitive detection of leukemia (single abnormal cells in ∼109 normal cells) in a blood serum. The sensitivity obtained by the SGEs was three orders of magnitude better than that of the best available and current technologies (e.g., specific mutations by polymerase chain reaction with detection limit of one abnormal cell in ∼106 normal cells) which not only are expensive, but also require several days for incubation. Significant variations in DPV signals of the SGEs after the first electrochemical cycle indicated that the best performance of the SGEs can be achieved only at the first cycle. The linear dynamic detection behavior of the SGEs was investigated in wide concentration range of 1.0×105–0.1cell/mL. The lower detection limit was estimated ∼0.02cell/mL, based on the current resolution obtained by the SGEs.

Determination of atropine sulfate using a novel sensitive DNA–biosensor based on its interaction on a modified pencil graphite electrode

A novel, selective, rapid and simple electrochemical method is developed for the determination of atropine sulfate. UV–Vis and differential pulse voltammetry are used to study the interaction of atropine sulfate with salmon sperm ds-DNA on the surface of salmon sperm ds-DNA modified-pencil graphite electrode (PGE). For this purpose, a pencil graphite electrode (PGE) modified with multiwall carbon nanotubes (MWCNTs), titanium dioxide nanoparticles (TiO2NPs), and poly-dialyldimethylammonium chloride (PDDA) decorated with ds-DNA is tested for the determination of atropine sulfate. The electrochemical oxidation peak current of adenine and guanine bonded on the surface of ds-DNA/PDDA–TiO2NPs–MWCNTs/PGE is used to obtain the analytical signal. Decreases in the intensities of guanine and adenine oxidation signals after their interaction with atropine sulfate are used as indicator signals for the sensitive determination of atropine sulfate. Using ds-DNA/PDDA–TiO2NPs–MWCNTs/PGE and based on the guanine signal, linear calibration curves were obtained in the range of 0.6 to 30.0μmolL−1 and 30.0 to 600.0μmolL−1 atropine sulfate with low detection limits of 30.0nmolL−1. The biosensor shows a good selectivity for the determination of atropine sulfate. Finally, the applicability of the biosensor is evaluated by measuring atropine sulfate in real samples with good accuracy.

DNA electrochemical biosensor for metallic drugs at physiological conditions

Entrapment of dsSS-DNA into the polypyrrole–polyvinyl sulfonate (dsSS-DNA–PPy–PVS) film over indium-tin-oxide (ITO) coated glass has been designed to detect titanium and platinum drugs, titanocene dichloride and cisplatin. The disposable dsSS-DNA–PPy–PVS/ITO biosensor was characterized by cyclic voltammetry, attenuated total reflectance Infrared spectroscopy and atomic force microscopy. Amperometric studies by cyclic voltammetry using, dsSS-DNA–PPy PVS/ITO biosensor, demonstrated the ability of this biosensor to detect these metallic drugs in millimolar concentration by monitoring the decrease of the guanine oxidation signal as a result of the DNA damage. The concentration range detected for titanocene dichloride is 0.25–1.5mM and for cisplatin is 0.06–1.0mM.

A glassy carbon electrode modified with graphene quantum dots and silver nanoparticles for simultaneous determination of guanine and adenine

A glassy carbon electrode (GCE) was modified with graphene quantum dots (GQDs) carrying silver nanoparticles. The modified GCE displays excellent performance in the electrochemical oxidation of guanine and adenine in showing lower anodic peak overpotentials (of 0.625 and 0.929 V, respectively) and increased anodic peak currents. The effective surface area of the modified GCE is about 21.5 times larger than that of the bare GCE. The electron transfer coefficient (α) and the electron transfer number (n) were calculated to be 2 and 0.60 for guanine, and 2 and 0.64 for adenine, respectively. Linear relationships between peak current and the concentrations were obtained in the range from 0.015 to 430 μM (for guanine) and 0.015 to 390 μM (for adenine). The detection limits are 10 nM and 12 nM (at a signal-to-noise ratio of 3), respectively. The modified GCE can well distinguish between guanine and adenine in mixed solutions, has fast response, a low detection limit, good reproducibility, and high stability.

Arabidopsis ZDP DNA 3'-phosphatase and ARP endonuclease function in 8-oxoG repair initiated by FPG and OGG1 DNA glycosylases.

Oxidation of guanine in DNA generates 7,8-dihydro-8-oxoguanine (8-oxoG), an ubiquitous lesion with mutagenic properties. 8-oxoG is primarily removed by DNA glycosylases distributed in two families, typified by bacterial Fpg proteins and eukaryotic Ogg1 proteins. Interestingly, plants possess both Fpg and Ogg1 homologs but their relative contributions to 8-oxoG repair remain uncertain. In this work we used Arabidopsis cell-free extracts to monitor 8-oxoG repair in wild-type and mutant plants. We found that both FPG and OGG1 catalyze excision of 8-oxoG in Arabidopsis cell extracts by a DNA glycosylase/lyase mechanism, and generate repair intermediates with blocked 3'-termini. An increase in oxidative damage is detected in both nuclear and mitochondrial DNA from double fpg ogg1 mutants, but not in single mutants, which suggests that a single deficiency in one of these DNA glycosylases may be compensated by the other. We also found that the DNA 3'-phosphatase ZDP (zinc finger DNA 3'-phosphoesterase) and the AP(apurinic/apyirmidinic) endonuclease ARP(apurinic endonuclease redox protein) are required in the 8-oxoG repair pathway to process the 3'-blocking ends generated by FPG and OGG1. Furthermore, deficiencies in ZDP and/or ARP decrease germination ability after seed deteriorating conditions. Altogether, our results suggest that Arabidopsis cells use both FPG and OGG1 to repair 8-oxoG in a pathway that requires ZDP and ARP in downstream steps.

New metal based drugs: Spectral, electrochemical, DNA-binding, surface morphology and anticancer activity properties

The NSAID piroxicam (PRX) drug was used for complex formation reactions with Cu(II), Zn(II) and Pt(II) metal salts have been synthesized. Then, these complexes have been characterized by spectroscopic and analytical techniques. Thermal behavior of the complexes were also investigated. The electrochemical properties of all complexes have been investigated by cyclic voltammetry (CV) using glassy carbon electrode. The biological activity of the complexes has been evaluated by examining their ability to bind to fish sperm double strand DNA (FSFSdsDNA) with UV spectroscopy. UV studies of the interaction of the PRX and its complexes with FSdsDNA have shown that these compounds can bind to FSdsDNA. The binding constants of the compounds with FSdsDNA have also been calculated. The morphology of the FSdsDNA, PRX, metal ions and metal complexes has been investigated by scanning electron microscopy (SEM). To get the SEM images, the interaction of compounds with FSdsDNA has been studied by means of differential pulse voltammetry (DPV) at FSdsDNA modified pencil graphite electrode (PGE). The decrease in intensity of the guanine oxidation signals has been used as an indicator for the interaction mechanism. The effect of proliferation PRX and complexes were examined on the HeLA and C6 cells using real-time cell analyzer with four different concentrations.

The study of adenine and guanine electrochemical oxidation using electrodes modified with graphene-platinum nanoparticles composites

Composite materials based on graphene and platinum nanoparticles (Gr-Pt-2 and Gr-Pt-3) were synthesized over two catalysts (Ptx/MgO, where x=2 or 3wt.%) using radio-frequency catalytic chemical vapor deposition (RF-CCVD), with methane as carbon source. After morphological (TEM/HRTEM) and structural characterization (XRD, XPS, FTIR and UV-Vis) they were used to modify two gold electrodes and subsequently employed for the investigation of adenine and guanine electrochemical oxidation. For the Au/Gr-Pt-2 electrode, the oxidation peak potential of adenine was observed at +1.19V vs. Ag/AgCl, while, for Au/Gr-Pt-3 electrode, this was negatively shifted to +1.09V vs. Ag/AgCl. In addition, the oxidation current densities were approximately 2.7×10−4 and 6.9×10−4 A·cm−2 (for Au/Gr-Pt-2 and Au/Gr-Pt-3, respectively) demonstrating that the Gr-Pt-3 composite had a better electro-catalytic activity towards the oxidation of adenine. A similar behavior was observed for guanine oxidation. The excellent electro-catalytic properties of the Gr-Pt-3 sample were correlated with the fact that the composite material had a higher amount of platinum nanoparticles which were not fully covered by graphene layers (about 50%). In addition, it has a larger surface area (335 m2·g−1) compared with that of the Gr-Pt-2 sample (271 m2·g−1), which also greatly improved the electron kinetics.

Electrocatalytic oxidation and electrochemical detection of guanine,l-arginine andl-lysine at a copper nanoparticles-modified electrode

The electrocatalytic oxidation of two amino acids, L-arginine and L-lysine, and the purine base guanine at a copper nanoparticles-modified carbon paste electrode (NCPE) was investigated. The results were also compared with a copper microparticles-modified carbon paste electrode (MCPE). In the voltammograms recorded using these electrodes, one anodic oxidation peak related to the electrocatalytic oxidation of the analytes (guanine, L-arginine and L-lysine) appeared through an electrocatalytic mechanism (an electrochemical reaction followed by a chemical reaction regenerating the initial redox species, EC′). For the electrooxidation of guanine, L-arginine and L-lysine, the catalytic rate constants were obtained as 5.5, 2.98 and 0.56 cm3 mol−1 s−1, respectively, the transfer coefficients were obtained as 0.59, 0.43 and 0.46, respectively, and the diffusion coefficients were obtained as 1.93, 1.54 and 3.28 × 10−6 cm2 s−1, respectively. These analytes were oxidized on NCPE with higher rates at low potentials compared to MCPE. This behavior was related to the nanosize effect of copper nanoparticles. Finally, a simple, sensitive and time-saving amperometric procedure was developed for the analysis of the analytes in both batch and flow systems. Using the amperometric procedure, guanine, L-arginine and L-lysine were quantified with linear ranges of 12.5–62.5, 20–263 and 50–1100 μM, respectively, calibration sensitivities of 5.8, 4.8 and 4.3 × 10−2 μA M−1, respectively, and limits of detection of 3.7, 4.3 and 8.6 μM, respectively, in batch system. In addition, for the flow system, guanine, L-arginine and L-lysine were quantified with linear ranges of 1–50, 50–700 and 100–1000 μM, respectively, calibration sensitivities of 4.4, 3.7 and 3.1 × 10−2 μA M−1, respectively, and limits of detection of 0.23, 20.0 and 29.1 μM, respectively.

Disposable electrochemical DNA biosensor for environmental monitoring of toxicant 2-aminoanthracene in the presence of chlorine in real samples

A simple procedure for the voltammetric detection of the DNA damage using a disposable electrochemical DNA biosensor is reported. The DNA biosensor is assembled by immobilizing the double stranded calf thymus DNA (dsDNA) on the surface of a disposable carbon screen-printed electrode. The interaction of 2-aminoanthracene (2-AA) with calf thymus dsDNA was studied electrochemically based on the oxidation signals of guanine (G) and adenine (A) by using square wave voltammetry (SWV) at screen printed electrode (SPE). The oxidation signals of the guanine and adenine bases, obtained by a square wave voltammetric scan, were used as analytical signal to detect the DNA damage. The presence of this aromatic amine compound with affinity for nucleic acids was measured by its effect on the guanine and adenine oxidation peaks. The response was obtained in the range of 0.05–20 mg L−1 for 2-AA concentration on dsDNA-modified SPE. This test has been used due to its rapid, easy handling and cost effective responses for the toxicity assessment in real water and bleach solution samples. A low-cost screen-printed DNA biosensor was developed as a simple strategy for the voltammetric detection of the DNA damage. The natural electrochemistry of guanine and adenine bases was exploited to develop the DNA-based biosensor.

In situ electrochemical assessment of cytotoxicity of chlorophenols in MCF-7 and HeLa cells

An in situ electrochemical method was used to assess the cytotoxicity of chlorophenols using human breast cancer (MCF-7) and cervical carcinoma (HeLa) cells as models. On treatment with different chlorophenols, the electrochemical responses of the selected cells, resulting from the oxidation of guanine and xanthine in the cytoplasm, indicated the cell viability. In addition, the in situ in vitro electrochemical method was further compared with the traditional MTT (3-(4,5-dimethylthiazol-2-yl)-2,5-diphenyltetrazolium bromide) assays. Although similar cytotoxicity data were obtained from both methods, the effective concentrations of chlorophenols that inhibited 50% cell growth (EC50 values) from the electrochemical method were only slightly lower than those from the MTT assay. These results indicate that the in situ in vitro electrochemical method paves a simple, rapid, strongly responsive, and label-free way to the cytotoxicity assessment of different chlorophenol pollutants.

Electrochemical biosensor for simultaneous determination of guanine and adenine based on dopamine-melanin colloidal nanospheres–graphene composites

Dopamine-melanin colloidal nanospheres (Dpa-melanin CNDs)–graphene composites-modified glassy carbon electrode (GCE) was prepared by a simple procedure and then successfully used to simultaneously determine guanine and adenine. Scanning electron microscopy (SEM) images and transmission electron microscopy (TEM) were used to characterize the morphology of the Dpa-melanin CNSs–graphene composite. Cyclic voltammetry (CV) and electrochemical impedance spectroscopy (EIS) were used to characterize the electrode modifying process. Differential pulse voltammetry (DPV) was used to study the electrocatalytic activity toward the electrochemical oxidation of guanine and adenine. The modified electrode exhibited enhanced electrocatalytic behavior and good stability for the simultaneous determination of guanine and adenine compared with bare GCE. The electrochemical biosensor exhibited wide linear range of 0.5 to 150 μM with detection limit of 0.05 and 0.03 μM for guanine and adenine detection (S/N = 3), respectively. Furthermore, the biosensor showed high sensitivity, good selectivity, good reproducibility, and long-term stability to guanine and adenine detection. At the same time, the fabricated electrode was successfully applied for the determination of guanine and adenine in denatured DNA samples with satisfying results. These results demonstrated that Dpa-melanin CNSs–graphene composite was a promising substrate for the development of high-performance electrochemical biosensor.